1. Field of the Invention
The present invention relates to an image forming apparatus configured as a multifunction product.
2. Description of the Related Art
Recently, multifunction image forming apparatuses are required to provide high quality images with high durability and high stability. That is, such an image forming apparatus needs to provide an image with little change in quality due to environmental changes, which is constantly stable over time. A two-component developing system is well known in which a two-component developer (hereinafter, “developer”) that contains a non-magnetic toner and a magnetic carrier is held on a developer carrier (hereinafter, “developing sleeve”), a magnetic brush is formed by magnetic poles included therein, and a developing bias is applied to the developing sleeve at a position facing a latent image carrier to develop a latent image. The two-component developing system is widely used because of its easiness of colorization. In this system, the two-component developer is conveyed to a developing region with the rotation of the developing sleeve. While the developer is conveyed to the developing region, magnetic carriers in the developer are concentrated together with toners along magnetic lines of a developing pole to form the magnetic brush.
In the two-component developing system, differently from a one-component developing system, it is preferential to precisely control a weight ratio of toners to carriers (toner concentration) to thereby improve stability. For example, when the toner concentration is excessively high, background stain occurs on an image or resolving power decreases at detailed parts. When the toner concentration is low, the concentration of a solid image portion lowers or carrier adhesion occurs. Therefore, it is necessary to control the amount of toner to be supplied to adjust the toner concentration in the developer within an appropriate range.
The toner concentration is controlled by comparing an output value Vt from a toner concentration detector (a permeability sensor) and a reference value Vref of a toner concentration, and setting the toner supply amount based on the comparison result.
A general method of detecting toner concentration uses a permeability sensor, in which permeability variation of the developer due to changes of the toner concentration is compared with a reference concentration to detect current toner concentration. Another toner concentration detecting method uses an optical sensor, in which a reference pattern is formed on an image carrier or on an intermediate transfer belt, reflection densities of an image portion and a non-image portion on the pattern are detected by the optical sensor, and toner concentration is detected based on the detection result. Besides, a method is also known in which a reference pattern is formed between sheets of paper to sequentially control a reference value Vref of a permeability sensor during printing. However, it is required to reduce, as much as possible, excessive consumption of toner caused by formation of a pattern between sheets, and there is a tendency not to perform correction based on the reference pattern formed between the sheets. When a pattern is formed on the intermediate transfer belt, a cleaning unit has to be arranged above a secondary transfer roller. Thus, in view of mechanical cost reduction, formation of a pattern between the sheets should be suppressed as much as possible. Accordingly, it is necessary to perform further accurate toner concentration control during continuous printing or at an image mode change (change of a process linear velocity) time using the permeability sensor alone.
In the two-component developing system, particularly, in a color image forming apparatus, an external additive such as silica or titanium oxide is applied to the toner surface to improve toner dispersibility. However, since the additive is susceptible to mechanical stress or thermal stress, they can be buried in toner, or separated from the surface during stirring in the developing system. As a result, fluidity or charging characteristic of the developer (including toner and carrier) and bulk density change. Further, the fluidity and the bulk density change due to decrease in chargeability (CA) of carrier caused by change in shape of the carrier surface, separation or accumulation of the external additive from and to toner, or carrier coat film wear with time.
These changes become a bottleneck for precisely detecting toner concentration by the permeability sensor. For example, in a system where a rotation speed of a stirring screw in the developing system changes according to image output modes based on a plurality of linear velocities, linear velocity shift takes place in which an output value changes even with the same toner concentration. One known approach to this problem is to previously obtain a linear velocity shift amount ΔVt from experiment data to be used as a correction amount in toner supply control. However, when the correction amount changes depending on a degradation state or a usage state of the developer, it is difficult to perform accurate corrections.
Japanese Patent Application Laid-Open No. 2002-207357 discloses a technique in which toner concentration in a developing device is detected by a toner concentration detector (a permeability sensor), and the detection value is compared with a threshold value to control the toner concentration. The threshold value for the detection value obtained by the toner concentration detector is changed according to change in linear velocity of a photosensitive drum. According to the technique, however, although it is considered to be possible to perform control at the initial stage, correction for degradation over time is not taken into consideration. Therefore, it is difficult to maintain stability over a long period of time.
Japanese Patent Application Laid-Open No. 2002-14588 discloses a technique for changing a threshold value of a toner concentration sensor according to a rotation speed of a developing device (a conveying screw). However, in this technique, correction for degradation over time is not taken into consideration as in the technique described above. Thus, it is also difficult to maintain stability over a long period of time.
Japanese Patent Application Laid-Open No. 2003-280355 discloses a technique of using a toner concentration sensor (a permeability sensor) value Vt for toner concentration control. However, in the case of this technique, Vcnt (T sensor control voltage) is changed for arranging Vt values. Characteristics (sensitivity) of a sensor may change largely by change in the Vcnt, Vcnt cannot be easily changed. Additionally, it is necessary to adjust the Vcnt to achieve a target Vt value while a voltage is changed over about ten points with, for example, a dual-partitioning approach, and considerable time is required for the adjustment. Further, a toner concentration needs to be set to a reference value (8%) during the adjustment, which increases the time required for process control.